The present invention relates to thermal printers and, more particularly, to a thermal printer for printing a plurality of colors (three colors in the currently preferred embodiment) on individual sheets of print receiving media--for example, sheets of paper--fed by a sheet feeder and utilizing a rotating drum with a clamp for holding and rotating the sheet past the thermal print head a plurality of times in the same direction and then reversing direction to release and eject the printed sheet.
Thermal printers are a rapidly emerging technology for printing in color at low cost. The typical prior art approach to thermal printers is shown in FIGS. 1-7. As shown in FIG. 1, the basic elements of a prior art thermal printer 10 are a rotatable platen 12 very similar to the platens employed in a standard typewriter. A paper feed guide 14 and pressure roller 16 are disposed adjacent the platen 12 to be employed in a manner to be discussed shortly. A printed circuit thermal print head such as that indicated as 18 in FIG. 2 is disposed close adjacent the surface of the platen 12 and parallel thereto. Print head 18 has a plurality of addressable vertical wires 20 and a single, horizontal wire 22 formed therein adjacent the surface closest to the platen 12. By sending current through the horizontal wire 22 and one of the addressable wires 20, the intersection of the two can be heated at a selected point along the length of the platen. A specialized print ribbon 24 as shown in FIG. 3 is disposed to pass from a supply roller 26 to a take up roller 28 passing over guide rollers 30 and passing between the print head 18 and platen 12. The print ribbon 24 has three colors in repeatable sequence designated as 32, 34, and 36. Typically, the three colors are magenta, yellow and cyan. The ribbon 24 is a thin backing strip which faces the print head 18 having the colors 32, 34, 36 as a thin heat-transferrable wax coating on the side facing the platen 12. The ribbon 24 in, for example, a printer designed to print standard 81/2.times.11 sheets of paper has each of the color segments approximately thirteen inches long disposed on a ribbon which is approximately eight and one-quarter inches wide.
The sequence of operation is shown in FIGS. 4-7. A sheet of paper 38 (single sheet or fan fold) is fed into the space between the platen 12 and paper feed guide 14 in the direction of arrow 40 as shown in FIG. 4. Pressure roller 16 forces the paper 38 against the surface of platen 12 which is then rotated (manually as in the case of a typewriter) to position the paper 38 at a top of paper position adjacent the print head 18. The print ribbon 24 is then positioned with the top of the first color 32 positioned at the top of the paper sheet 38. A mechanism (not shown) then forces the print head 18 against the ribbon 24, paper 38, and platen 12 in the direction of arrow 42 as shown in FIG. 5. The horizontal wire 22 and addressable vertical wires 20 of the print head 18 are then addressed sequentially across the print head 18 according to a control signal providing a line of print information from a computer or the like (not shown) to cause the individual pixel positions across the platen 12 at the intersection of the various wires 20, 22 to heat and cause the wax of first color 32 to be melted into the paper 38 along the first line to be printed. The longer a single pixel position is heated, the more wax is transferred to the paper 38 and, consequently, the bigger the dot of color at that position. These are all techniques well known in the art and are provided for general information purposes only. As shown in FIG. 6, the print head 18 is then withdrawn from the contact and print position of FIG. 5 to the withdrawn position as indicated by the arrow 44. The ribbon 24 is then advanced in the direction of arrow 46 while the paper 38 is advanced one line by rotating the platen 12 as indicated by the arrows 48. Initially, the ribbon 24 and paper 38 remain stuck together due to the melting of the wax surface. As the advancing of the ribbon 24 and paper 38 continues, however, the two are pulled apart. When the paper 38 and ribbon 24 have been advanced one line, the print action as described above with reference to FIG. 5 is repeated. The ribbon 24 and paper 38 are then advanced once again as described with respect to FIG. 6. This print and feed sequence of FIGS. 5 and 6 is repeated until the first color 32 has been completely printed on the sheet of paper 38. As shown in FIG. 7, the paper 38 is then rewound in the direction of arrows 50 by rotating the platen 12 in the opposite direction until the top of the paper sheet is adjacent the print head 18 corresponding once again to the position of FIG. 4. The ribbon 24 is adjusted as necessary to place the second color 34 in alignment for printing. The sequence of FIGS. 5 and 6 is then repeated to print the second color. The paper sheet 38 is then rewound in the manner of FIG. 7 once again and the third color 36 of ribbon 24 aligned whereupon the FIGS. 5-6 sequence is repeated for a third time to print the third color.
As will be realized, this procedure is time consuming in that the paper sheet 38 must be rewound to the top of sheet twice. Additionally, there is a registration problem in that the colors 32, 34, 36 do not always align identically line by line and pixel position by pixel position such that color ghosting occurs on the document as printed.
In another prior art approach of which the assignee of the present has recentlly become aware (See U.S. Pat. Nos. 4,388,628 and 4,496,955), a thermal printer drum is rotated a plurality of times in the same direction to facilitate multi-color thermal transfer from an ink donor sheet onto a recording sheet or media.
In the '628 patent, the lengths of each color ink layer on the ink donor sheet are equal to the outer circumference of the printer drum (purportedly to insure that a single color ink is applied to the sheet during each rotation of the drum). A clip is provided on the drum to grasp the receiving sheet and is released so that a stripping finger can separate the receiving sheet from the drum. Details of operation of the clip and of the stripping finger are not disclosed, nor is there discussion of the ultimate disposition and handling of the receiving sheet as it is being stripped from the drum. Further, there is no discussion regarding a mechanism for preventing misregistration (which might occur because of stretching or contraction of the ink donor sheet or because of relative "slippage" of the drum with respect to the ink donor sheet over a plurality of printing operations) of the length of ink with the leading edge of the receiving sheet.
In the '955 patent, index strips are provided adjacent each individual color frame on a web on one said thereof and at the beginning of each sequence of a set of colors on the other side of the web. Sensors are provided to detect the positions of the index strips to permit adjustment of the positions of the donor sheet or web and insure registration of the individual color frames with the sheets on which images are to be printed and to permit the donor sheet to be rewound to the start of the color sequence to permit successive multi-color printing operations to be performed using the same set of color frames. Indicia and corresponding sensors are provided to permit detection of when the printer drum is in its "home" or "start" position and in its "print" position. This patent apparently does not address itself to releasing the image receiving media.
Wherefore, it is the object of the present invention to provide a thermal printer which is simple, reliable and faster in printing by eliminating the rewinding step while, simultaneously, assuring registration on the various passes of color printing and simply and reliably effecting the release of media upon which multi-color images have been printed.